This invention pertains to thin metallic film resistors for use in electronic circuits, and methods for producing such resistors.
Electrical resistors are normally comprised of a relatively low conductivity material, contacted by two metal electrodes and supported by an insulative substrate. A thin film resistor is one in which the resistive material is a film whose thickness usually ranges from a few hundred angstroms to a few thousand angstroms, which material is commonly carbon, metal, or metal oxide.
Of the many methods that can be used for producing thin resistive metallic films, those of evaporation or sputtering in a vacuum are generally preferred over electrolysis or pyrolysis, the latter being prone to leave contaminants in the film which adversely affect the stability and quality of the film product.
Evaporation of metals or metal alloys is accomplished in a vacuum, usually less than one micron, by heating a metal that evaporates and condenses on a nearby substrate. Variations in evaporation technique are mainly related to the method of heating and evaporating the metal, some typical heating methods being by electron beam, laser, resistive heating, radio frequency inductive heating, and flashing a metal coating from a tungsten wire by application of a large step of electrical current through the wire.
Sputtering is the ejection of atoms from the surface of a material by bombardment with energetic particles, which particles are normally positive ions. A common source of ions for sputtering is provided by the well-known phenomenon of glow discharge due to an applied electric field between two electrodes in a gas at low pressures. Another common source of ions for sputtering is the ionization of a low pressure gas by exposure to a high intensity radio frequency (r.f.) electromagnetic field which may be accomplished by the application of an r.f. voltage to electrodes, or r.f. current through a coil in the vacuum chamber. Many practical variations of these two methods of ion production are employed. The sputtered atoms from the ion bombarded material condense on a nearby substrate forming a thin film.
The rate of deposition of atoms on a substrate by sputtering is generally more readily controlled than by evaporation. The sputtered film is also generally more uniform.
The present invention relates particularly to the production of thin resistive films of nickel-chromium alloys. Among other metals and alloys the nickel-chromium alloys have a relatively high bulk resistivity, around 100 .times. 10.sup.-.sup.6 ohm-cm, and are thus capable of providing higher value resistors in a given space. They also possess a relatively low temperature coefficient of resistance, typically 300 ppm/.degree. C. The alloy known as Nichrome; 60 Ni, 24 Fe, 16 Cr, 0.1C; has been favored for many years as a resistor material. Nickel rich alloys of around 80 Ni and 20 Cr have been used extensively in thin film resistors.
Evaporation methods as opposed to sputtering are usually chosen for the production of nickel chromium resistors due to the relative simplicity of the production equipment, its operation, and maintenance. A large batch of resistors may be deposited by this method having a spread in resistance values of within 10% of the mean. However, the temperature coefficients of these same resistors typically may vary 600 ppm/.degree. C. within 95% of the distribution. The center of the distribution will usually lie between 200 and 400 ppm/.degree. C. depending upon the composition of the evaporating material, the temperature at evaporation, the rate of evaporation, the ultimate film thickness and other factors.
This broad spread of temperature coefficients of resistance within a batch of evaporated thin film resistors, is believed to be attributable in large measure to a broad spread of nickel-chromium compositions. A standard phase diagram for the alloys of nickel and chromium reveals that compositions richer than about 70% nickel and poorer than about 12% nickel are stable single phase alloys at room temperature. They are readily realizable by a variety of noncritical production means, for use at normal room temperature. However, compositions in the middle range are known as metastable and are achieved with difficulty, for use at room temperature, for example by fast quenching such as splat cooling from the liquid state above 1300.degree. C. Slow cooling of such compositions from the liquid state yields a mixture of stable phases. In recent studies it has been found that metastable alloys may also be achieved by codeposit quenching, (See page 202, Thin Film Phenomena by Kasturi L. Chopra, McGraw Hill Book Co. 1969) using either evaporation or sputtering. Sputtering however is greatly preferred, giving more uniform and predictable metastable material. After deposition, the film is annealed at a temperature of about a third of the mean of the melting temperatures of the pure metals. The annealing process yields an irreversible single phase metastable alloy. Much is yet to be learned of the properties of metastable alloys.
Heretofore, when close control of the temperature coefficient of resistance (TCR) is desired, a nickel chromium film can be sputtered yielding a nominal TCR of 150 .+-. 30 ppm/.degree. C. Furthermore the sputtered film temperature coefficients are not nearly so strong a function of process variables or film thickness as for the evaporated films. See for example "Characterization of Vacuum Evaporated and Sputtered Nickel-Chromium Films" by I. H. Pratt, pp. 215-220, Proceedings of the National Electronics Conference, 1964, Vol. 20.
However, when it is desired to produce nickel chromium resistors having a near zero TCR then the choice must be to sort out the few resistors from a batch of evaporated nickel-chromium resistors that have this characteristic. The effort of sorting and the uncertain but always low yields for such resistors makes them very expensive. The few evaporated resistors that are found to have near zero TCR also are found to be chromium rich and are at least partly composed of metastable nickel-chromium alloy. Heretofore there has been no method to produce such resistors with high yields.
It is therefore an object of this invention to provide a method for making with high yields and low cost, thin film nickel chromium resistors having near zero temperature coefficients of resistance.